The invention relates to a polymerization process to obtain sulphonic fluorinated ionomers with high productivity.
Specifically, the invention relates to a polymerization process in emulsion to obtain sulphonic fluorinated-ionomers, said process characterized by high productivity and by operating conditions such to minimize the coagulum formation during the polymerization and the reactor corrosion.
The polymerization of fluorinated monomers in emulsion is known in the prior art. However the productivity data are much lower compared with the polymerization in microemulsion by using ammonium perfluorooctanoate which is the industrially commonly used surfactant for the polymerization in aqueous emulsion of fluorinated monomers. It shows a series of drawbacks with respect to the polymerization in microemulsion. Higher polymerization yields are obtained by using the microemulsion.
The need was therefore felt to have available a polymerization process to obtain sulphonic fluorinated ionomers, said process characterized by high productivity, absence of coagulum formation during the polymerization and minimal reactor corrosion.
An object of the present invention is a polymerization process in aqueous emulsion of:
(I) one or more fluorinated monomers containing at least one ethylene unsaturation;
(II) one or more fluorinated monomers containing sulphonyl groups xe2x80x94SO2F;
said process comprising:
reactor purging, monomer (II) introduction in liquid form into the reactor, reactor pressurization with gaseous monomers (I); addition of at least one surfactant of formula:
Rfxe2x80x94Xxe2x88x92M+
xe2x80x83wherein
X is equal to xe2x80x94COO, xe2x80x94SO3;
M is selected from H, NH4, alkaline metal;
Rf represents a (per)fluoropolyether chain, preferably having number average molecular weight comprised between about 230 and about 1,800, preferably from 300 to 750, said (per)fluoropolyether chain comprising repeating units selected from one or more of the following:a)
a) xe2x80x94(C3F6O)xe2x80x94;
b) xe2x80x94(CF2CF2O)xe2x80x94;
c) xe2x80x94(CFL0O)xe2x80x94, wherein L0=xe2x80x94F, xe2x80x94CF3;
d) xe2x80x94CF2(CF2)zxe2x80x2CF2Oxe2x80x94, wherein zxe2x80x2 is an integer 1 or 2;
e) xe2x80x94CH2CF2CF2Oxe2x80x94.
Rf is monofunctional, and has a (per)fluorooxyalkyl end group T, for example CF3Oxe2x80x94, C2F5Oxe2x80x94, C3F7Oxe2x80x94; optionally in the (per)fluoroalkyl end groups one fluorine atom can be substituted by one chlorine or hydrogen atom;
addition of the initiator and, during the polymerization, feeding of monomers (I) so as to maintain constant the reactor pressure; optionally further addition of monomer (H) and of chain transfer agents.
Preferably during the polymerization the monomer (II) is added by steps.
Examples of end groups containing chlorine or hydrogen atoms are Cl(C3F6O)xe2x80x94, H(C3F6O)xe2x80x94. The unit a) C3F6O is xe2x80x94CF2xe2x80x94CF(CF3)Oxe2x80x94 or xe2x80x94CF(CF3)CF2Oxe2x88x92.
In particular Rf has preferably one of the following structures:
1) Txe2x80x94(CF2O)axe2x80x94(CF2CF2O)bxe2x80x94CF2xe2x80x94 with b/a comprised between 0.3 and 10, extremes included, a being an integer different from 0;
2) Txe2x80x94(CF2xe2x80x94(CF2)zxe2x80x2xe2x80x94CF2O)bxe2x80x2xe2x80x94CF2xe2x80x94 wherein zxe2x80x2 is an integer equal to 1 or 2;
3) Txe2x80x94(C3F6O)rxe2x80x94(C2F4O)bxe2x80x94(CFL0O)txe2x80x94CF2xe2x80x94 with r/b=0.5-2.0 (r+b)/t=10-30, b and t being integers different from 0 when all the units with indexes r, b, and t are present; or b=t=0, or b=0;
a, b, bxe2x80x2, r, t, are integers, whose sum is such that Rf has the above values of number average molecular weight.
The compounds wherein Rf has the following formula are still more preferred:
Txe2x80x94(CF2CF(CF3)O)m(CF2O)nxe2x80x94CF2xe2x80x94
wherein m/n=1-30;
wherein T=xe2x80x94OCF3 or xe2x80x94OCF2Cl,
X is a carboxylic group and M is NH4, K.
Mixtures of one or more surfactants of the above classes can be used.
The (per)fluoropolyethers Rf are obtainable with the well known processes of the prior art, see for example the following patents herein incorporated by reference: U.S. Pat. Nos. 3,665,041, 2,242,218, 3,715,378, and the European patent EP 239,123. The functionalized fluoropolyethers with hydroxyl termination are obtained for example according to patents EP 148,482, U.S. Pat. No. 3,810,874, from which the functional groups X are obtained with the processes mentioned in said patents.
At the end of the polymer process a latex is obtained. The latex discharged from the polymerization reactor can be subjected, if desired, to the usual post-treatments depending on the planned uses, in particular on the coagulation and drying process.
The fluorinated monomers of type (I) are selected from:
vinylidene fluoride (VDF);
C2-C8 perfluoroolefins, preferably tetrafluoroethylene (TFE);
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;
(per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a C1-C6 (per)fluoroalkyl, for example trifluoromethyl, bromo difluoromethyl, pentafluoropropyl;
perfluoro-oxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is a C1-C12 perfluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.
The fluorinated monomers of type (II) are selected from:
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F (vinylsulphonylfluoride);
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94[CF2xe2x80x94CXFxe2x80x94O]nxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F wherein X=Cl, F or CF3; n=1-10;
F2Cxe2x95x90CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F;
F2Cxe2x95x90CFxe2x80x94Arxe2x80x94SO2F.
Optionally in the polymerization process of the present invention, besides the monomers of type (I) and (II) bis olefins of formula:
R1R2Cxe2x95x90CHxe2x80x94(CF2)mxe2x80x94CHxe2x95x90CR5R6xe2x80x83xe2x80x83(III)
can bemused as comonomers, in an amount in the range 0.01-5% by moles, wherein:
m=2-10, preferably 4-8;
R1, R2, R5, R6, equal to or different from each other, are H or C1-C5 alkyl groups.
Optionally in the polymerization process of the present invention brominated and/or iodinated xe2x80x9ccure-sitexe2x80x9d comonomers, can be fed, as bromo and/or iodo olefins having from 2 to 10 carbon atoms (as described for example in U.S. Pat. No. 4,035,565 and U.S. Pat. No. 4,694,045), or iodo and/or bromo fluoro-alkylvinylethers (as described in U.S. Pat. No. 4,745,165, U.S. Pat. No. 4,564,662 and EP 199,138), in such amounts whereby the xe2x80x9ccure-sitexe2x80x9d comonomer content in the final product is generally comprised between 0.05 and 2 moles per 100 moles of the other basis monomeric units.
Alternatively or also in combination with the xe2x80x9ccure-sitexe2x80x9d comonomers, it is possible to introduce in the end groups iodine and/or bromine atoms by addition to the reaction mixture of iodinated and/or brominated chain transfer agents, such for example the compounds of formula Rf(I)x(Br)y, wherein Rf is a (per)fluoroalkyl or a (per)fluorochloroalkyl having from 1 to 8 carbon atoms, while x and y are integers comprised between 0 and 2, with 1xe2x89xa6x+yxe2x89xa62 (see for example U.S. Pat. No. 4,243,770 and U.S. Pat. No. 4,943,622). It is also possible to use as chain transfer agents iodides and/or bromides of alkaline or alkaline-earth metals, according to U.S. Pat. No. 5,173,553. Other usable chain transfer agents are molecules containing hydrogen, such as hydrocarbons, alcohols, in particular ethyl acetate and ethane.
The polymerization initiators used in the process of the present invention are organic or inorganic. As organic initiators, IPP, DTBP, etc. can for example be mentioned. Preferably radical inorganic initiators, such for example the ammonium and/or potassium and/or sodium persulphate, optionally in combination with ferrous, cupreous or silver salts, are used. The initiator feeding procedures can be in a continuous way or by a single addition at the polymerization starting.
Preferably in the polymerization process of the present invention are used as comonomers:
TFE;
CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2SO2F;
optionally a bis-olefin of formula (III), wherein R1, R2, R5, R6 are H and m=6 or 8.
With the polymerization process of the invention it is possible to obtain sulphonic fluorinated ionomers having a high molecular weight and a good chemical and thermal stability. However it is possible to reduce the molecular weight by using chain transfer agents as above mentioned.
With the polymerization process of the invention it is possible to obtain sulphonic fluorinated ionomers having both high and low equivalent weight, in particular comprised between 300 and 1,700. The equivalent weight can decrease till to 280 when the preferred sulphonic monomer is homopolymerized.
The polymerization reaction is generally carried out at temperatures in the range 25xc2x0-130xc2x0 C., at atmospheric pressure or under pressure, for example from 2 bar up to 60 bar.
Preferably the polymerization reaction is generally carried out at temperatures in the range 50xc2x0-70xc2x0 C., preferably 50xc2x0-60xc2x0 C., under pressure up to 30 bar, preferably higher than 8 to obtain high molecular weights.
The sulphonic fluorinated ionomers obtained from the invention process are suitable to be used in the preparation of membranes for fuel cells, of membranes for electrochemical applications, such for example chloro-soda cells, lithium batteries, and electrodialysis, and in reactors in which the ionomeric membrane acts as a superacid catalyst.
The results obtained with the present invention are surprising and unexpected, since high productivities of the order of those obtainable with the microemulsions are obtained, without the drawbacks related to the use of the microemulsions. However the microemulsion use requires a supplementary unit for their preparation and the use of various components for their obtaining. Some of said components remain in the final polymer and to remove them supplementary polymer treatments are required.
Furthermore the productivity data obtained are surprisingly and unexpectedly much higher with respect to the ammoniumperfluorooctanoate which is the industrially commonly used surfactant for the polymerization in aqueous emulsion.